The invention relates generally to light-emitting diodes and more particularly to AlGaInN-based light-emitting diodes.
Light emitting diodes (LEDs) are well known solid state devices that can generate light having a peak wavelength in a specific region of the light spectrum. LEDs are typically used as illuminators, indicators and displays. Aluminum-gallium-indium-nitride (AlGaInN)-based LEDs can emit light having a peak wavelength in the blue and green region of the visible light spectrum with greater luminous intensity than other conventional LEDs. Due to their superior luminance, the AlGaInN-based LEDs have attracted much attention in recent years.
An exemplary prior art AlGaInN-based LED is schematically illustrated in FIG. 1. The AlGaInN-based LED 10 includes a sapphire (Al2O3) substrate 12 and a multi-layered epitaxial structure 14. The multi-layered epitaxial structure includes an upper AIGaInN region 16, an active region 18, and a lower AlGaInN region 20. The term xe2x80x9cAlGaInNxe2x80x9d is defined herein as a material composed of one or more elements from a set that includes aluminum, gallium, indium, and nitrogen. The upper AlGaInN region and the lower AlGaInN region are made of multiple epitaxial layers of AlGaInN. The upper AlGaInN region is made of p-type AlGaInN epitaxial layers, while the lower AlGaInN region is made of n-type and/or undoped AlGaInN epitaxial layers. The active region includes one or more layers of indium-gallium-nitride (InGaN) that form(s) what is/are commonly referred to as quantum well(s). The typical thickness of the upper AlGaInN region is less than 0.5 micrometers, while the typical thickness of the lower AlGaInN region is approximately 2 to 3 micrometers. The active region thickness is typically less than 0.2 micrometers. Therefore, the typical maximum thickness of the multi-layered epitaxial structure is approximately 3.7 micrometers.
An ohmic p-contact 22 is connected to the upper AlGaInN region 16, while an ohmic n-contact 24 is connected to the top surface of the lower AlGaInN region 20. The ohmic contacts provide electrical conduction through the active region 18 of the multi-layered epitaxial structure 14. Located on the top surface of the upper AlGaInN region and electrically connected to the ohmic p-contact is a semi-transparent metallic layer 26. The semi-transparent metallic layer may be made of oxidized nickel and gold. The semi-transparent metallic layer functions as a current spreader to distribute current throughout the area of the active region. Situated between the sapphire substrate 12 and the conductive structure is a buffer layer 28. The buffer layer serves as a transition layer to ensure proper growth of the lower AlGaInN region on the sapphire substrate. The buffer layer is made of AlGaInN-based material.
During a light-emitting operation, voltage is applied to the ohmic contacts 22 and 24 to forward bias the LED 10. The forward biased condition of the LED causes holes to be injected into the active region 18 from the upper AlGaInN region 16. Furthermore, the forward biased condition causes electrons to be injected into the active region from the lower AlGaInN region 20. Within the quantum well(s) of the active region, the injected holes and electrons recombine, which results in emission of light. The generated light is emitted in all directions, as illustrated by the arrows in the active region. Some of the emitted light escapes the LED from the top of the LED through the semi-transparent metallic layer as output light. Also, some of the light emits into the substrate 12 and escapes out the sides of the device. However, much of the emitted light must escape from within the multi-layered epitaxial structure 14 out the sides of the LED, after reflecting from the upper surface of the sapphire substrate and the lower surfaces of an encapsulation epoxy layer (not shown) that covers the LED. This is illustrated by an exemplary path 30 of such emitted light. The overall light output of the LED includes the portion of the emitted light that escapes through the sides of the multi-layered epitaxial structure, as well as the portion that escapes through the top surface and through the substrate.
Light extraction from AIGaInN-based LEDs, such as the LED 10, is limited by the various parasitic optical loss mechanisms present within or surrounding the AlGaInN epitaxial layers. These mechanisms include absorption at the semi-transparent metallic layer 26, as well as absorption within many layers comprising the epitaxial portion of the LED, such as the buffer layer 28, the active region 18 and the heavily Mg-doped GaN contact layer 16. Because of the refractive index step between the multi-layered epitaxial structure (n=2.4) and the sapphire substrate (n=1.77) or the encapsulation epoxy layer (n=1.5), only approximately 25% of the light generated within the active region escapes into the epoxy or the substrate upon first encountering these interfaces. The rest of the light is trapped in a waveguide formed by the encapsulation epoxy layer above the chip and the substrate. The trapped light must travel distances on the order of the length of the chip to escape from the sides of the LED. Such distances require many passes through the various loss mechanisms within the LED structure, increasing the probability of absorption. Thus, much of this trapped light is eventually lost, decreasing the overall light output of the LED.
Therefore, what is needed is an AlGaInN-based LED structure that reduces the amount of emitted light that is lost to the various loss mechanisms, thereby increasing the overall output of the device.
A light-emitting diode (LED) and a method of making the device utilize a thick multi-layered epitaxial structure that increases the light extraction efficiency of the device. The LED is an aluminum-gallium-indium-nitride (AlGaInN)-based LED. The thick multi-layered epitaxial structure increases the light extraction efficiency of the device by increasing the amount of emitted light that escapes the device through the sides of the thick multi-layered epitaxial structure.
The LED includes a substrate, a buffer layer, and the thick multi-layered epitaxial structure. The substrate is preferably made of sapphire. Since the improvements with increased epitaxial thickness on light extraction will be manifested as long as the substrate or an overgrowth layer on the substrate, if any, has an refractive index of appreciably less than that of the multi-layered AlGaInN epitaxial structure (nxcx9c2.4 effectively), other substrates are possible. For the purposes of this teaching, significant improvements in light extraction are expected for different substrates, provided that the refractive index of the substrate or the overgrowth layer is less than 2.1. Also, the substrate may support layers of zinc-oxide, silicon-dioxide, or another dielectric material to provide particular characteristics during growth. In the preferred embodiment, the upper surface of the substrate is textured to randomize the light that impinges upon the textured surface. As an example, the surface may be mechanically textured by polishing the surface with a relatively coarse grinding grit. The buffer layer is formed over the substrate by epitaxially growing a layer of AlGaInN-based material. The buffer layer serves as a transition layer to ensure proper growth of the multi-layered epitaxial structure upon the substrate.
The multi-layered epitaxial structure includes an upper AlGaInN region, an active region, and a lower AlGaInN region. The upper and lower AlGaInN regions include multiple epitaxial layers of AlGaInN. The upper AlGaInN region is made of p-type AlGaInN epitaxial layers, while the lower AlGaInN region is made of n-type AlGaInN epitaxial layers and undoped epitaxial layers. The undoped epitaxial layers may be layers of AlGaInN or other AlGaInN-based material. The active region includes at least one AlGaInN light-emitting layer that forms the quantum well(s). For visible spectrum LEDs, the light-emitting layer is typically comprised of InGaN. The multi-layered epitaxial structure is thicker than conventional multi-layered epitaxial structures. The multi-layered epitaxial structure has an approximate thickness of 4 micrometers or greater. In one embodiment, the thickness of the multi-layered epitaxial structure is approximately 7 micrometers. In another embodiment, the thickness of the multi-layered epitaxial structure is approximately 15 micrometers. However, the thickness of the multi-layered epitaxial structure may be greater than 15 micrometers.
The increased thickness of the multi-layered epitaxial structure allows trapped light within a waveguide, formed by the upper surface of the substrate and the lower surface of an encapsulating epoxy layer that covers the LED, to escape the LED through the sides of the multi-layered epitaxial structure with fewer reflections. In particular, the increased thickness of the multi-layered epitaxial structure allows a greater amount of emitted light to exit from the sides of the multi-layered epitaxial structure with a single reflection from either the upper surface of the substrate or the lower surface of the encapsulation epoxy layer. The decrease in the number of reflections for the trapped light reduces the amount of light that is absorbed by various parasitic optical loss mechanisms present within or surrounding the AlGaInN epitaxial layers. The effect of this reduction is an overall increase in the light output of the LED.